FIG. 14 shows the basic construction of a previously known imaging probe. The imaging probe 150 consists of a catheter 152, a piezoelectric transducer 154 (i.e., a transducer having a material that electrically polarizes when mechanically strained and that mechanically strains when electrically polarized) at the distal end 168 of catheter 152 (i.e., the end of the catheter that goes into the body), electric wires 156 that connect piezoelectric transducer 154 to external circuitry at the proximal end (i.e., the end that stays outside the body), an acoustic reflector 158 (i.e., a mirror), a rotating drive shaft 164 coupled to a small motor/shaft encoder at the proximal end, an acoustic reflector 158 at the distal end, and a plastic radome 160 (i.e., an acoustic window that has the same acoustic impedance as a fluid) filled with a liquid 162 that fits over piezoelectric transducer 154 and acoustic reflector 158.
Piezoelectric transducer 154 is stationary and when excited by an external source, it produces an acoustic signal 166 that travels through the liquid in radome 160 and strikes acoustic reflector 158. The surface of acoustic reflector 158 resides at an angle of 45.degree. from acoustic signal 166 and it reflects acoustic signal 166 at an angle of 90.degree. from its original path. The reflected acoustic signal 166 travels through liquid 162 in radome 160 and propagates through the blood until it encounters the artery wall.
Depending on the penetration, several echoes return to piezoelectric transducer 154 by retracing essentially the same path. Piezoelectric transducer 154 converts these echoes into corresponding electrical pulses. And wires 156 carry these electrical pulses to electrical circuitry located at the proximal end. Since acoustic reflector 158 continuously rotates, acoustic signal 166 continuously rotates. Echoes from each angular position are collected, processed and displayed on a CRT screen.
Previously known imaging probes have variations of the configuration shown by FIG. 14. In an alternate configuration, the acoustic reflector 158 and piezoelectric transducer 154 exchange places. These and other configurations have the following in common: all use a piezoelectric transducer at the distal end of the catheter, which goes inside the body.
Placing the piezoelectric transducer at the distal end of the catheter that goes inside the body has numerous disadvantages. The piezoelectric transducer may emit leakage currents inside the body that can induce fibrillation when the probe images a coronary artery. Wires 156 that connect the piezoelectric transducer to external circuitry inherently act as antennas and they receive radio frequency (RF) interference present in a catheterization laboratory. This RF interference appears as noise in the electrical signals that travel to and from the piezoelectric transducer and increases the risk that these electrical signals may cause fibrillation.
Another disadvantage of placing the piezoelectric transducer at the distal end of the catheter that goes inside the body is that the piezoelectric transducer has a frequency of operation determined by its thickness. The user cannot adjust the piezoelectric transducer frequency to obtain a more desirable resolution or to illuminate a particular region of interest.
Another disadvantage of placing the piezoelectric transducer at the distal end of the catheter is that after one use the piezoelectric transducer must be discarded along with the catheter to prevent the transmission of disease. This is burdensome because the piezoelectric transducers are difficult and expensive to make. It also discourages use of the most desirable transducers because they usually are more expensive. Generally, increasing the frequency of the acoustic waves improves the resolution capability of the transducer, but it also increases the expense because the output frequency of piezoelectric transducers depends upon their thickness (i.e., a 40 MHz piezoelectric transducer would have a thickness of approximately 2 mils) and the thinner the transducer, the more expensive the transducer.